测

Left indexing three-drops water radical — left-side variant of 水. Originally 测 specifically meant 'to measure water depth,' using a marked rod or sounding line. From the concrete sense came the abstract 'to measure, gauge, predict' (测试 test, 预测 forecast). Anchors 测 in the water family alongside 深 (deep), 浅 (shallow), 流 (flow).

Right 则 supplies the sound (zé → cè through Old Chinese z/c alternation). 则 itself means 'rule, standard, regulation,' and lends a strong semantic flavor: measurement is application of a standard. So 则 is partly phonetic, partly reinforcing the meaning of 'measuring against a rule.' Same phonetic family: 侧 (side), 厕 (toilet — euphemism).